May 14, 2012

The US Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) has
released a new repeatable test protocol that simulates real shade conditions and can
predict with much greater precision the effects of shade on a solar array.

The new test demonstrated that under heavy shading conditions the use of microinverters
instead of typical string inverters can help mitigate the impacts of shade by improving
system performance by more than 12 percent.

“Photovoltaic (PV) Shading Testbed for Module-level Power Electronics” was co-authored
by NREL senior engineers Chris Deline and Jenya Meydbray, as well as Jason Forrest
and Matt Donovan of PV Evolution Labs of Davis, Calif. The research was paid for by
DOE.

Shade significantly impacts photovoltaic performance, and is considered in PV system
design. The effects of shade can vary depending on the configuration of the PV modules,
the extent of the shade, and the use of shade mitigating power electronics in the
system. The industry currently lacks representative, repeatable test procedures for
evaluating the annual effect of shade on different PV systems equipped with different
shade mitigation devices.

The new report details a repeatable test procedure for simulating shaded operation
of a PV system and an analysis model for converting these measurements into annual
performance forecasts.

Shade measurements from more than 60 residential installations provide the basis for
the shading conditions employed during the test, which are analyzed for three typical
shade scenarios: “light”, “moderate”, and “heavy” shading. The relative performance
of a system using shade mitigation devices is compared against an identical system
equipped with a reference string inverter for these three shade scenarios, providing
an annual performance improvement score.

Combined with additional derates like annual shade loss and inverter CEC efficiency,
this annual shade improvement score can allow performance modeling software such as
PV Watts http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/ and System Advisor Model https://sam.nrel.gov/ to better predict annual performance for PV systems that use shade mitigating power
electronics. It also allows an accurate comparison between different devices.

An initial application of the test protocol was conducted by PV Evolution Labs, showing
the shaded performance benefit of microinverters compared with a typical string inverter
on identical 8-kW solar arrays. The microinverter was found to increase system production
by 3.7 percent under light shading, 7.8 percent under moderate shading, and 12.3 percent
under heavy shading, relative to the reference string inverter case. Additional detail
is provided in the report to allow duplication of the test method for different power
electronics devices and test installations.

Standard test methodologies using applicable test conditions should provide value
to the PV community, since products can be compared by a common metric and accurate
information can be collected about devices’ annual performance benefit.

This is a major step in establishing new and realistic testing standards for PV power
electronics,” said David Briggs of Enphase Energy, a microinverter manufacturer.

NREL is the U.S. Department of Energy's primary national laboratory for renewable
energy and energy efficiency research and development. NREL is operated for DOE by
the Alliance for Sustainable Energy, LLC.